Blocking SMTP connections
If a VPS user has been found to be spamming other users, you can block them temporarily from accessing SMTP ports by issuing commands similar to the following on the node that their VPS is situated on:
iptables -t mangle -I POSTROUTING -s <IP ADDRESS> -p tcp --source-port 25 -j DROP iptables -t mangle -I POSTROUTING -s <IP ADDRESS> -p tcp --destination-port 25 -j DROP